3D printing of piezoelectric ceramics with tailored grain orientation

This thesis presents a comprehensive investigation into the fabrication of textured piezoelectric ceramics using two additive manufacturing (AM) (also known as 3D Printing- 3DP) techniques: Direct Ink Writing (DIW) and blade-assisted Vat Photopolymerization (VPP). Piezoelectric ceramics, vital for applications like actuators, sensors, ultrasound transducers, and energy harvesters, benefit significantly from texturing—aligning crystallographic orientations to enhance electromechanical performance by exploiting material anisotropy. The research in this thesis was conducted on potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN), a material of significant interest as a lead-free alternative.

This thesis, to the best of our knowledge, presents the first piezoceramics with texture aligned parallel to the DIW print direction. This was achieved through the use of needle-like KNN templates and templated grain growth (TGG), by optimizing processing parameters, developing matrix and template inks with tailored rheological properties and printability, aligning the templates during the printing process, and achieving TGG during heat treatment. The results demonstrated that DIW can achieve a high degree of texturing (Lotgering factor of 84 %), with the alignment direction closely following the printing path. The textured KNN ceramics exhibited significant enhancements in piezoelectric properties, including a 41 % increase in the piezoelectric charge constant (d₃₃) and an 88 % increase in the field-induced strain (d*₃₃) compared to non-textured samples. This innovative approach not only allows to harness the full potential of anisotropy by applying texture in various directions but also enables the production of intricate geometries with enhanced and extended properties, previously unattainable with conventional manufacturing and AM of randomly oriented polycrystalline piezoelectric ceramics.

The fabrication of textured KNN was also explored using VPP-based AM. Photopolymerizable slurries were initially formulated using two types of KNN powders, synthesized via solid-state and spray pyrolysis methods. The findings revealed that the characteristics of the KNN powders significantly influenced processing parameters, including the rheological behavior of the slurries, as well as the final piezoelectric response of the VPP-processed KNN ceramics. These results provide a foundation for developing slurries for KNN-based ceramics from various sources. Furthermore, a blade-assisted VPP system was employed to fabricate textured KNN. Templated photopolymerizable slurries were developed using NaNbO₃ platelet templates, and their rheological behavior was compared with that of the matrix slurries. The texturing results demonstrated that blade-assisted VPP, combined with heteroepitaxial TGG, can produce high-quality texture (Lotgering factor of 96 %), comparable to texture achieved by conventional tape casting. The textured samples exhibited a 49 % improvement in d₃₃ and a 22 % increase in d*₃₃, with further enhancements observed after additional densification through cold isostatic pressing (CIP). The study also highlights the influence of poling conditions on piezoelectric performance, with textured samples consistently outperforming their randomly oriented counterparts.

This thesis demonstrates a framework for producing textured KNN with enhanced electromechanical properties while enabling the creation of intricate geometries. These advancements open new possibilities for applying AM to a wider range of advanced ceramics, including other lead-free piezoelectric ceramics, and contribute to the development of new piezoelectric devices where grain orientation and shape can be simultaneously tailored.